Cisco IE 4000, Software Configuration Manual

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Configuring IP Multicast Routing
Guidelines and Limitations

If you have non-Cisco PIMv2 routers that need to interoperate with Cisco PIMv1 routers and multilayer switches, both
Auto-RP and a BSR are required. We recommend that a Cisco PIMv2 device be both the Auto-RP mapping agent
and the BSR. For more information, see
Using Auto-RP and a BSR, page 709 .
PIM Stub Routing Configuration Guidelines
Guidelines and limitations for PIM stub routing are as follows:

Before configuring PIM stub routing, you must have IP multicast routing configured on both the stub router and the
central router. You must also have PIM mode (dense-mode, sparse-mode, or dense-sparse-mode) configured on
the uplink interface of the stub router.

The PIM stub router does not route the transit traffic between the distribution routers. Unicast (EIGRP) stub routing
enforces this behavior. You must configure unicast stub routing to assist the PIM stub router behavior.

Only directly connected multicast (IGMP) receivers and sources are allowed in the Layer 2 access domains. The PIM
protocol is not supported in access domains.

The redundant PIM stub router topology is not supported.
Restrictions for Legacy Applications Within the SSM Range
Existing applications in a network predating Source-Specific Multicast (SSM) do not work within the SSM range unless
they are modified to support (S, G) channel subscriptions. Therefore, enabling SSM in a network can cause problems for
existing applications if they use addresses within the designated SSM range.
Address Management Restrictions
Address management is still necessary to some degree when SSM is used with Layer 2 switching mechanisms. Cisco
Group Management Protocol (CGMP), IGMP snooping, or Router-Port Group Management Protocol (RGMP) support only
group-specific filtering, not (S, G) channel-specific filtering. If different receivers in a switched network request different
(S, G) channels sharing the same group, they do not benefit from these existing mechanisms. Instead, both receivers
receive all (S, G) channel traffic and filter out the unwanted traffic on input. Because SSM can re-use the group addresses
in the SSM range for many independent applications, this situation can lead to decreased traffic filtering in a switched
network. For this reason, it is important to use random IP addresses from the SSM range for an application to minimize
the chance for re-use of a single address within the SSM range between different applications. For example, an
application service providing a set of television channels should, even with SSM, use a different group for each television
(S, G) channel. This setup guarantees that multiple receivers to different channels within the same application service
never experience traffic aliasing in networks that include Layer 2 switches.
IGMP Snooping and CGMP Limitations
IGMPv3 uses new membership report messages that might not be correctly recognized by older IGMP snooping
switches.
For more information about switching issues related to IGMP (especially with CGMP), refer to the “Configuring IGMP
Version 3” section of the “Configuring IP Multicast Routing” chapter.
State Maintenance Limitations
In PIM-SSM, the last hop router continues to periodically send (S, G) join messages if appropriate (S, G) subscriptions
are on the interfaces. Therefore, as long as receivers send (S, G) subscriptions, the shortest path tree (SPT) state from
the receivers to the source is maintained, even if the source does not send traffic for longer periods of time (or even
never).
This case is opposite to PIM-SM, where (S, G) state is maintained only if the source is sending traffic and receivers are
joining the group. If a source stops sending traffic for more than 3 minutes in PIM-SM, the (S, G) state is deleted and only
re-established after packets from the source arrive again through the RPT. Because no mechanism in PIM-SSM notifies
a receiver that a source is active, the network must maintain the (S, G) state in PIM-SSM as long as receivers are
requesting receipt of that channel.